When structural materials are exposed to particular aggressive service environments under steady or cyclic stress, the material will be susceptible to damage in the form of cracking. This type of damage is commonly referred to as "stress corrosion cracking" or "corrosion fatigue" (hereinafter "SCC"). It is desirable to monitor and assess the extent of damage to structural components due to SCC, for example, in a nuclear plant which has been operating for a number of years to help predict its lifetime. Methods for directly measuring crack growth in specimens removed from their environment are known. These methods use a variety of monitoring systems including visual and voltage potential drop methods.
U.S. Pat. No. 4,677,855 to Coffin et al. discloses a method for accurately assessing crack growth in structural components through voltage potential drop methods by establishing a reasonably accurate relationship between measured voltage and crack size. This patent discloses a sensor for measuring growth of a preformed crack within a solid exposed to an aggressive environment during application of a load. The crack is defined as possessing a mouth and a tip. The "mouth of the crack" is defined as the point or line of action of load application. The "crack tip" is the leading edge of the crack. The "length" of the crack is defined as the distance from the mouth of the crack to the crack tip. The preformed crack within the solid is of a known length. The size and shape of the preformed crack can vary widely; however, the cracks cannot be so large that the sensor will be separated into two sections.
The solid must be electrically conductive, such as carbon or alloy steel, nickel and nickel-based alloys, titanium and its alloys and structural materials such as austenitic stainless steels and the like. A current is passed through the solid to establish a voltage drop across the crack. This voltage is measured by at least two pairs of probes, the probes of each pair being positioned on opposite sides of the crack at equal, known distances from the mouth of the crack.
When a current is caused to flow through the sensor perpendicular to the crack, the potential difference between two points located on opposite sides of the crack will increase as the size of the crack increases. Measurement of the electric potential will provide information as to the instantaneous damage as well as the accumulated damage to the sensor in the form of crack growth. The measured voltage across each probe pair is plotted versus the distance from the crack mouth. A "best fit" curve or straight line of voltage versus the distance of the probe pairs from the crack mouth is drawn through these points and extrapolated to obtain the x-intercept, i.e., when the voltage will be zero. X-intercept values thus obtained are used to calculate the length of a propagating crack.
The sensor provides information which reflects on the condition of a particular structural component of interest. To achieve this purpose, it is preferable to manufacture this sensor from the same material with the same process history as the structural component of interest. When monitoring the damage to a structural component, the sensor is placed within the same environment as the structural component. The sensor then experiences the same changing environmental conditions as these structural components. The sensor is disposed in the aggressive environment, i.e., an environment which attacks the sensor material with sufficient severity to enhance the growth of the preformed crack, and the crack is supplied with a stress intensity at the crack tip which correlates to a stress intensity which the structural component experiences under operating conditions.
Although the sensor size, crack size and crack location can vary widely, the configuration of the sensor preferably permits a load of sufficient magnitude to be applied to the crack to provide a crack tip stress intensity factor that will allow the crack to grow at an appropriate rate. A sensor having a double-cantilever beam ("DCB") geometry permits a load of sufficient magnitude to be conveniently applied. As shown in FIG. 1, a sensor 10 with DCB geometry has two parallel arms (beams) 12 and 14 joined at one end and separated at the other. A slot or deep notch 16 separates the arms, and the base of this notch is referred to as the notch root 18. The preformed crack 20 is preferably positioned at the notch root. This configuration permits a number of measurements to be taken at various positions along the two beams 12, 14 since the effective crack length is extended along these beams. In addition, if the load remains constant, the stress intensity factor at the crack tip increases as the distance between the crack tip and the point of the load increases. Therefore, the long length of the sensor permits the threshold crack tip stress intensity to be obtained at low load levels.
Side grooves 34 placed within the sensor along the plane of the preformed crack determine the plane in which the crack grows. It is important to keep the fracture surfaces of the crack as planar as possible to avoid multiple cracking and bridging of the crack. Bridging can lead to a short circuit in the current flow and cause errors in the electric potential measurements.
For monitoring SCC in aggressive environments, an active load or a fixed displacement must be applied to the sensor. The means for applying a fixed displacement to cause the preformed crack to grow can be wedge 24 forced within the notch to expand the crack or other suitable means such as a clamp or bolt. The means for applying a fixed displacement must be made of electrically non-conductive material.
Crack growth is preferably monitored by measuring a potential or voltage across pairs of probes disposed along beams 12 and 14, and using such measured voltages, as well as the initial parameters, to calculate a crack length. Calculated crack lengths may advantageously be plotted as a function of time in order to assess a rate of crack growth. At least two pairs of probes are used to detect the voltage across the crack. However, at least three pairs of probes 26a/26b, 28a/28b and 30a/30b are preferred for accurate measurement of the crack growth. Each pair of probes is positioned at a different distance from the mouth of the crack, indicated in FIG. 1 as X.sub.1, X.sub.2 and X.sub.3, respectively. The two probes of each pair are positioned on opposite sides of the crack, preferably an equal distance from the plane of the crack. The two probes of each pair are also equidistant from the mouth of the crack, i.e., they are the same distance from the leads 32a and 32b which supply a current to the sensor.
The sensor is supported in the aggressive environment by pressure coupling 36. Channel 38 provides access to channels (or holes) 40, both of which provide pathways for the conductive leads attached to the probe pairs and to conductive leads which preferably supply a d.c. potential to the sensor. The reversing direct current is supplied at points 32a and 32b and the effective initial length of the crack is indicated by dimension a.sub.O.